1. Field of the Invention
The present invention relates to a laminated optical film, a polarizing plate including the laminated optical film, and a liquid crystal display device using the polarizing plate.
2. Description of the Related Art
Liquid crystal display devices are provided with optical films (phase difference films) having optical anisotropy, for the purpose of enlarging viewing angles, for instance. The transmittance of the liquid crystal display devices can be controlled by adjusting the optical properties (e.g. in-plane retardation values) of these optical films; thus, there are demands for a wider range of optical properties of the optical films.
Accordingly, the range of optical properties of the optical films is widened by lamination of the optical films.
Since optical films of this type are made of transparent polymer films, they are easily affected by temperature. Thus, when the temperature changes, their optical properties may also change.
For instance, when a back light of a liquid crystal display device is on, an optical film receives heat and increases in temperature, and thus the optical properties of the optical film may change. Also, after the back light has been turned off, the optical film decreases in temperature, and thus the optical properties of the optical film may change.
If the optical properties of the optical films change, it is problematic because the display quality of liquid crystal display devices changes as well.
Especially in the case where a plurality of optical films are laid on top of one another, there is a problem that a greater effect is caused by a change in optical property that is due to a change in temperature, as the number of optical films increases.
Also, in recent years, liquid crystal display devices have been used for car navigation systems and display units for vehicles, e.g. instrumental panels, which are strongly required to be durable against changes of use environments such as temperature conditions. Thus, provision of optical films which are durable against temperature change, in particular, is hoped for.
For instance, Japanese Patent Application Laid-Open (JP-A) No. 2006-39211 discloses a technique for reducing change in optical property that is due to dimensional change, by setting dimensional change directions of two phase difference plates at right angles.
However, this technique is problematic because change in the optical properties of a film in its thickness direction cannot be reduced.